This invention relates to the art of processing video signals and, more particularly, to improvements in recursive filters.
Recursive filters are known in the art and an example takes the form of the U.S. patent to A. Kaiser et al., U.S. Pat. No. 4,064,530. The recursive filter described there serves to reduce noise in a color television signal, even in the presence of motion between successive frames. The system includes a delay or frame storage device for storing a single television frame and a summing device for adding a fractional amplitude portion of the stored signal to a fractional amplitude portion of the present or incoming video signal. The system functions as a recursive filter and is operative automatically to change the fractional amplitude portion of the stored signal fed back to the summing device as a function of the difference between the stored and present signals. This changes the integration time constant of the filter so as to accommodate a certain amount of motion between the arriving signal and the stored frames. Motion is detected as it exists between stored frames and the incoming signal as the picture proceeds element-by-element through the system, and in response to the evaluation of such motion alters the contribution of the stored past signals to the noise reduced video output signal. If a picture element from the same scene object in the stored past signals is sufficiently different in amplitude from the same element in the arriving video signal, the past history of that picture is ignored and only the present signal is transmitted to the output terminal.
In the Kaiser et al. system, supra, no provision is made for adjusting the fractional amplitude portion of the stored signal which is fed back into the summing device as a function of noise as well as a function of the difference between the stored and the present signals. To the contrary, Kaiser et al. provides only for adjustment of the fractional amplitude portion of the stored signal as a function of the difference between the stored and the present signals.
It is, however, known to provide a noise measurement circuit in conjunction with a video noise reduction system as is described in the patent to R. Storey et al., U.S. Pat. No. 4,249,210. Storey contemplates a somewhat different filtering than that of Kaiser, supra. In Storey et al., a signal is derived from a preceding output signal and is then subtracted from the input signal for the current field to provide a difference signal. Low amplitude portions of the difference signal are attenuated relative to the high amplitude portions thereof. The thus attenuated signal is added to the preceding output signal to provide a new output signal for the current field. While the system operates differently than that of Kaiser, Storey et al. nevertheless contemplates that a noise measurement circuit be employed for measuring the noise on the difference signal and then adjusting the gain of a variable gain element employed in attenuating the difference signal as a function of the output obtained from the noise measurement circuit.